The present invention relates to fiber optic attenuators for use in single-channel applications, and more particularly to the use of crosslinked siloxane polymers as refractive index matched materials in such devices.
Dense wavelength division multiplexed (D/WDM) optical networks increase their transmission capacity by employing multiple co-propagating, discrete, wavelength channels, each carrying independent data streams. Currently, D/WDM systems operate in the 1550 nm spectral region because of the availability of optical amplifiers containing erbium-doped optical fibers (i.e. Er+3 fiber). However, as amplifier technology develops, and capacity demands increase, D/WDM systems are expected to expand their spectral extent and increase their channel density.
Broadband fiber optic devices, such as variable attenuators, couplers, and switches having a controllable spectral response, are critical components of D/WDM systems. The availability of dispersion and refractive index controlled polymers can be used to develop side-polished fiber- (SPF) and tapered fiber-based device designs. Examples of such devices and polymers are disclosed in copending commonly assigned U.S. application Ser. Nos. 09/139,787, 09/139,832 and 09/139,457, all filed on Aug. 25, 1998.
In contrast to broadband devices, which are designed to cover a spectral region of tens of nanometers, single-channel or narrowband devices are used in DJWDM systems where single channels are separated out. Certain polymers having a specific refractive index associated therewith at any given single wavelength can be incorporated into such narrowband devices. Classes of polymers, such as polyolefins and polysiloxanes, have excellent optical properties making them ideal for use as optical materials.
In U.S. Pat. Nos. 5,217,811 and 5,266,352, Filas et al. disclose optical devices in which certain crosslinked silicone polymers are incorporated to index match optical components, such as the end of a broken optical fiber, to other optical devices. The polymer disclosed is a vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer crosslinked with tri- or tetrafunctional silanes in the presence of a platinum catalyst. Examples of refractive index matching applications include coupling the end of one broken optical fiber to another or into a light detector, optical matching a lens to the end of the broken fiber, connecting the end of an optical fiber to a waveguide, and coupling energy from the waveguide into a PIN photodetector. In all these examples, the optical fiber is broken, and the polymer is positioned in series between the components, such that the optical signal is transmitted through the polymer without alteration.
As disclosed in commonly assigned U.S. Pat. No. 5,966,493 and the aforementioned copending commonly assigned patent applications, certain organic polymers having an index of refraction close to that of the fiber can be applied to the side surface of a SPF optic or tapered fiber for use in variable optical attenuators (VOAs). Such attenuators lower the optical signal levels of light transmitted therethrough, without the need to break the optical fiber. The disclosed polymers exhibit a change in refractive index proportional to a change in temperature. OPTI-CLAD(copyright) 145, which is available from Optical Polymer Research, Inc. is an example of such a polymer. Polyolefin/dye compositions, wherein the refractive index varies with polymer composition as well as with temperature, are disclosed in the aforementioned copending commonly assigned U.S. application Ser. Nos. 09/139,457 and 09/139,787. These polymers can be used in broadband applications to give a uniform spectral response, but because of the inclusion of the dye, the polyolefin compositions are limited to use in the 1500-1600 nm wavelength region.
A need therefore exists for a polymer composition and optical gel that can attenuate light within a single channel. Furthermore, a polymer composition is desirable wherein the refractive index (xcex7) can be controlled by changing the temperature of the polymer or by controlling the polymer composition. Such a polymer gel should exhibit good thermal stability, good attenuation control at operating temperatures varying over a wide range, very low dxcex7/dT, low power consumption, and low polarization dependent loss. The present invention meets the aforementioned needs.
Accordingly, in one aspect, the present invention is a single-channel attenuator comprising a portion of an optical fiber through which optical energy can propagate, wherein the portion has a side surface through which at least some of the optical energy can be extracted. Overlying the side surface of the optical fiber is a crosslinked siloxane polymer composition, which is the product of crosslinking an uncrosslinked vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer with a silane crosslinking agent in the presence of a platinum catalyst. The optical energy propagating through the fiber can be controllably extracted therefrom at a particular wavelength by adjusting the refractive index of the crosslinked siloxane polymer composition. Preferably, it should substantially match the refractive index of the optical fiber. As used herein, the term xe2x80x9csubstantiallyxe2x80x9d matched refers to within about 0.5%, and the term xe2x80x9coptical fiberxe2x80x9d is interchangeable with the term xe2x80x9cfiber opticxe2x80x9d.
In addition, the attenuator may optionally include a controllable heating/cooling source in contact with the crosslinked siloxane polymer composition (and optionally with the uncrosslinked copolymer to induce curing) wherein the controllable heating/cooling source provides a controllable stimulus to change the temperature of the crosslinked polymer composition, thereby adjusting the refractive index until it substantially matches that of the optical fiber.
In another aspect, the invention is a method for preparing a vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer having a desired mole percent of phenyl monomers. The method comprises redistributing monomers from a first vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer and a second vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer. The method comprises the steps of:
(a) mixing the first vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer having a higher mole percent of phenyl monomers with the second vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer having a lower mole percent of phenyl monomers in the presence of a trace amount of an alkaline hydroxide selected from the group of KOH and NaOH; and
(b) heating the mixture of step (a) at a temperature and for a time sufficient to produce a clear homogenous solution.
The clear solution contains the vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer having a desired mole percent of phenyl monomers lying between the higher and lower mole percent.
In yet another aspect, the invention is a fiber optical device for attenuating optical energy (i.e. attenuator) within a single channel, as set forth above. However, the vinyl-terminated dimethylsiloxane-diphenylsiloxane copolymer, which is subsequently crosslinked, is prepared by the redistribution method of the present invention.